1. Field of the Invention
This invention relates to temperature measuring equipment, specifically to an apparatus and method for determining temperature in an open environment by measuring variations in the speed of sound.
2. Description of the Prior Art
The speed of sound within any medium is dependent on the temperature of the medium. Therefore, the time required for sound to traverse a predetermined distance through the medium can be used in determining temperature along the sound's path. This basic principle is common to all relevant prior art, and is the basis of the present invention. However, implementation methods and application areas vary considerably.
Much of the existing art is directed toward fashioning probe-like sensors in which an enclosed sound path is created within a relatively compact apparatus. The enclosed sound path is often defined by a wave guide structure or a solid rod-like device. The wave guide is generally defined by a closed cavity filled with a measurement medium (sometimes air). U.S. Pat. No. 5,123,750 to Ziegler et al. (1992) discloses a cavity guide structure with sound reflecting walls. Rod-like devices generally employ a probe made of a solid material through which sound vibrations are sent. U.S. Pat. No. 3,514,747 to Lynnworth et al. (1970) and U.S. Pat. No. 3,580,076 to Mobsby (1971) are examples of using solid materials as sound guides. In both the cavity guide and solid guide cases, the measurement path is confined within the dimensions of the probe. These closed probe devices measure only the temperature of the probe itself. Such devices are intended for use within unusual environments (i.e. at very high temperatures or in radioactive areas). At normal room temperatures and atmospheric conditions they have no advantage over conventional thermometers.
In U.S. Pat. No. 5,286,109 to Hanscombe et al. (1994) a solid sensor wire probe is used. This probe can be of considerable length; even traversing a building. However, the sound path is still confined within the probe. Measurements through an open area are not possible.
Other existing art devices are directed toward specific applications; as is the case in U.S. Pat. No. 4,848,924 to Nuspl et al. (1989). The apparatus of this patent is designed specifically for the measurement of high temperatures within a noisy environment, such as in a furnace flue. In this case the sound path is not confined to a wave guide. The medium traversed is a gas mixture within the flue. Sound travels from a transmitter located on one side of the flue to a receiver on an opposite side. The transmitter and receiver are wired to a single controller. The sound travels through the medium in only one direction. Movement of the medium (wind) in a direction non-perpendicular to the sound path can create large measurement errors. This severely limits the use of the Nuspl invention in areas where such medium movements can exist.
Conventional, non-acoustic, temperature sensors such as liquid and bimetal thermometers, thermistors, and thermocouples all share the limitation of measuring temperature at a single, confined location. An example where a single measurement point is not desirable is in a thermostatic control for the heating system for a large room; such as an auditorium. Although the desire is to achieve a comfortable temperature throughout the room, the conventional sensor provides the system with temperature information at only one point, usually close to a wall. Additional sensors can be used, but their placement is still only at the room's periphery, and combining their measurements can greatly complicate the system.
A need exists for a temperature measurement apparatus that can provide an average reading of temperature across an open area. The apparatus needs to be sufficiently accurate for its application. Further, it must not be prone to errors induced by environmental factors, such as movement of the measured medium.